Is the Chart Recorder sitting on or mounted to a surface prone to vibration?

For Temperature units try removing the unit from the application and placing it in an isolated, low air flow environment like a zip lock freezer bag or desk drawer. If the pen traces straighten out, the traces may have actually been caused by the changes in your application.

The chart recorder and digital display are actually two independent measuring instruments, with different specifications. There is certain amount of discrepancy allowed between chart and display, the amount of the discrepancy depends on the range of the chart recorder. An SL4120, for example, has a scale of -20 to +120 degrees, that's a range of 140 degrees. That means a 2.8 degree tolerance for the chart. The digital display has tolerance of 3.6 degrees, which means there is a maximum allowed discrepancy of 6.4 degrees between the chart and display. The chart may be adjusted using the pen adjust screw on the face of the unit. If the display is out of specification, the display can be replaced but not calibrated.

Remember that the unit's tolerance speaks to the Chart Recorder only, there is any additional that applies to the probe

Has an outside calibration house attempted calibration? It may not have been adjusted properly

When was the last time the unit was sent to the Dickson Calibration Lab for a recalibration?

Walk through zero adjustment, found in the manual

For units with Pen Adjustment - rotate screw to adjust the unit to match another unit of greater accuracy after both units have stablized in the same environment.

Chart Recorders - make sure the correct chart is being used.

K-Thermocouple UnitsK-Thermocouple capable units are capable of using any K-type thermocouple probe or extension but keep in mind that long extensions to extend the distance may require a Zero adjustment. (Refer to manual)
As a general rule: If the unit is not responding properly, and you have more than one thermocouple, swap probes to see if it is the probe or the unit that is defective.

Voltage Recorders

Make sure the output signal of the transmitter is calibrated to its full measuring range.

Make sure the range selected on the recorder matches the transmitter exactly and that the proper chart is being used.